Apparatus and method for fluorine production

ABSTRACT

Apparatus and a method for the generation of fluorine by the electrolysis of hydrogen fluoride are described. The apparatus comprises: a plurality of individual fluorine generating cassettes; said individual fluorine generating cassettes being operably connected to a fluorine gas distribution system for the remote use and consumption of said fluorine gas; said fluorine generating cassettes being individually isolatable from said gas distribution system and removable from the apparatus for remote maintenance.

The present invention relates to an apparatus and a method of using theapparatus for the production of fluorine.

Semiconductor devices, for example, are generally produced in vacuumprocess chambers by the chemical vapour deposition (CVD) of a pluralityof layers of silicon for example. The constituent layers of material arealso etched to provide a desired pattern on the device during itsmanufacture. Such etching in a vacuum chamber results in some of thesubstrate material being etched, e.g. silicon, silicon oxide, siliconnitride, for example, being deposited on the process chamber surfacesover time. Most of the unused chemical reagents and by-products of thedeposition or etching process are exhausted from the chamber at eachprocess step, however, some of these essentially unwanted reagents andby-products are inevitably deposited on the process chamber walls andsurfaces and become potential contaminants. For example, it is possiblefor some of these deposited materials to fall from the chamber walls andbecome incorporated in the devices themselves which would render them asscrap. Such unwanted deposits and residues must be periodically cleanedfrom the process chamber surfaces before building up to undesirable andpotentially harmful levels.

Although semiconductor devices have been specifically referred to above,CVD is a very widely employed technique for the production of many typesof electronic devices. For example, CVD is used in the production ofthin film transistors (TFT) flat panel displays by depositing films ofmaterial on large substrates such as glass panels, for example, in theproduction of liquid crystal displays (LCD).

Conventionally, the cleaning of undesired materials from the surfaces ofCVD process chambers was accomplished by use of cleaning gases such asnitrogen trifluoride, hexafluoroethane and sulphur hexafluoride. Whilstthese gases work well in removing contaminants from process chambersthey have the disadvantage of contributing to global warming if releasedinto the earth's atmosphere. These gases, in use, are decomposed byplasma means to release atomic fluorine in the process chamber and whichis the active cleaning constituent.

More recently it has been established that instead of the conventionalgas compounds referred to above, molecular fluorine gas may be usedeither directly or as atomic fluorine, after treating molecular fluorinein a plasma chamber, for CVD process chamber cleaning. Molecularfluorine has the added advantage that it does not contribute to globalwarming. EP-A-1 138 802 describes the use of molecular fluorine for theremoval of contaminant materials from CVD chambers.

However, whilst EP-A-1 138 802 discussed above makes it clear thatmolecular fluorine in itself or treated to produce atomic fluorine isefficacious in cleaning CVD chambers, there is no hint as to how suchgaseous fluorine may be generated to supply a commercial CVD plant.

The electronic devices manufacturing industry, however, is not wellequipped to either maintain or deal with the conventional chemical plantinstallations which would be required to generate fluorine in thequantities necessary for the cleaning of the number of CVD chamberswhich exist in a typical modern plant for the manufacture of electronicdevices. The industry demands reliable, easily expandable on-siteproduction and delivery of a high-purity fluorine gas stream. It isdesirable that maintenance and expansion of a fluorine generating plantbe carried out quickly and simply with minimal or no chemical hazard andimportantly, no loss of production during such maintenance or expansion.The aspect of expansion is important as some users will require thatfluorine generating capacity be increased as the required manufacturingcapacity of electronic devices increases due to increased numbers of CVDchambers.

The use of compressed fluorine in cylinders for large scale CVDapplications is not practical due to safety concerns and the fact thatthe maximum quantity of fluorine stored at 28 bar gauge is only 1.4 kgin a typical 50 litre cylinder. Thus, compressed fluorine in thequantities required at a normal commercial plant represent anunacceptable environmental and safety hazard as the reactivity offluorine increases greatly when compressed. Furthermore, the cost ofproviding large quantities of fluorine in such a manner would beprohibitive.

On-site generation of fluorine currently exists in some industries,however, the fluorine generating apparatus has been designed to suitchemical industry standards relating to fluorine generation and requiressignificant on-site maintenance and frequent operator intervention andoften involves handling or sampling of chemicals such as the electrolytefrom which the fluorine gas is produced. This procedure is common in thechemical industry where the expectation is that personnel will wearappropriate protective clothing and use breathing apparatus, forexample, to protect themselves from hazardous fluorine gas liberated toatmosphere when cell connections are broken or cells opened. However,this approach is not acceptable in the electronics industry where theexpectation is that personnel will not need to rely on protectiveclothing and the like and that any hazardous gases will be contained atall times and not liberated to atmosphere under any circumstances.

In the absence of a particular process benefit resulting from the use ofmolecular fluorine for CVD tool cleaning it has been expressed that acost saving of at least 30% would be required for the industry to changefrom the existing methods and cleaning chemicals. Thus, it wasoriginally considered that each CVD process tool might be coupled to aconventional fluorine generating cell. This approach has obviousapparent benefits in that the output of the fluorine cell may be matchedto the fluorine requirements of the particular tool which it supplies.Such an approach would also minimise fluorine process piping. A furtherapparent advantage is that failure of one fluorine generator would notresult in a plant-wide shutdown, only a shutdown of one CVD tool.However, in reality, the superficial attractions of one fluorinegenerator per CVD tool are outweighed by the practical and economicdisadvantages of such an approach. Each fluorine generator would have tohave the same process modules and would thus necessitate unnecessary andcostly duplication of components and services, including: anhydroushydrogen fluoride supply, downstream purification of the fluorine gas,gas compression and storage and generator effluent abatement. Thus, thisproliferation would result in the distribution of a relatively hazardouschemical process, including large hydrogen fluoride inventories andfluorine gas storage across an entire production plant. As a consequenceof this, all fluorine generator operation and maintenance activitieswould be similarly distributed which would have adverse implications forplant safety.

Additionally, fluorine gas quality is of paramount importance in CVDcleaning applications and gas quality control is much more difficult ascontinuous on-line analysis is prohibitively expensive when severalinstallations would be required and periodic fluorine gas sampling froma plurality of installations has many practical and safety drawbacks.

In contrast to the one fluorine generator per tool approach describedabove, the single; large conventional generator with separate feeds to aplurality of CVD tools has the overwhelming disadvantage that failure orshutdown of the generator for repair or maintenance results in theentire production process being halted for however long the generator isout of commission.

A first aspect of the present invention relates to apparatus for thegeneration of fluorine by the electrolysis of hydrogen fluoride, theapparatus comprising: a plurality of individual fluorine generatingunits; said individual fluorine generating unit being operably connectedto a fluorine gas distribution system for the remote use and consumptionof said fluorine gas; said fluorine-generating units being individuallyisolatable from said gas distribution system and removable from theapparatus for remote maintenance.

In the above, the word “maintenance” is intended to cover any reasonwhatsoever for the removal of the individual fluorine generating unitfrom the apparatus. In the present invention the word “maintenance” may,for example, include routine maintenance, servicing or repair of thatcell. It is further intended that such a cell requiring maintenance beremoved from the apparatus and taken to a remote site away from theplant where the fluorine is being consumed such that there is noinconvenience, contamination or safety hazard to the production plant orpersonnel.

In this specification the apparatus is a “packaged” fluorine generatingapparatus. The term “packaged” is intended to mean a plant which isbuilt and assembled at a fluorine plant supplier company, for example,tested to ensure efficient operation of the apparatus, closed down,sealed and then shipped as a self-contained plant to a customer site forexample, for use of the fluorine in the customer's processing.Generally, the apparatus may be totally self contained; apart fromcustomer provided services such as electricity, water, compressed air ornitrogen supplies, for example, to render the apparatus operative; in acontainer shippable by land or sea and that container being thecontainer in which the apparatus will reside and operate at thecustomer's site.

Packaged fluorine generating plant or apparatus may be generallycategorised as apparatus intended to produce in total from 0 to 2.7 kgof F₂ per hour. There exist conventional large scale fluorine generatingplants in the chemical industry which are capable of producing severalthousand tonnes of fluorine per year, where each cell will typicallygenerate not less than 4 kg of fluorine per hour, these plants beingable to have individual cells removed by personnel wearing and usingappropriate safety equipment, they are built in-situ for a plurality ofcells and associated equipment brought to site of operation andassembled. Such plants are used, for example, in the nuclear industryfor the production of nuclear fuel precursors such as uraniumhexafluoride, for example. Such plants are quite different and distinctfrom the “packaged”, transportable as a unit fluorine generatingapparatus forming the subject matter of the present invention. In thepresent invention the apparatus may be contained, for example, within acontainer having overall dimensions substantially not more than astandard ISO container or less as explained in greater detail below.

In this specification, the apparatus comprises a plurality ofself-contained fluorine generating units. The units may be constitutedby a single cell insofar as that unit has effectively one cathode andone anode. Alternatively, the unit may comprise a group of cells insofaras there may be more than one cathode and anode in that unit. Thus, theapparatus according to the present invention comprises a plurality offluorine generating units which are isolatable one from another and fromthe apparatus as a whole and each unit is individually removable fromthe apparatus without interrupting the supply of fluorine from theapparatus as a whole. In the interests of ease of description, anindividual fluorine generating unit will henceforth be termed a“fluorine cassette”. Similarly, other facilities in the fluorineproducing apparatus such as fluorine purification units and fluorinecompression and storage units are also termed “cassettes”, e.g.“purification cassette” and “compression and storage cassette”. The term“cassette” is intended to convey the meaning of a self contained packagehaving the described feature or facility which may be easily removedfrom the apparatus for maintenance or repair and be replaced by anidentical package without danger to people or detriment to the smoothproduction of fluorine.

It is common terminology in the fluorine generating industry to refer toa fluorine “cell” as one metal container vessel but which vessel maypossess a plurality of anodes therein (the container itself normallyconstitutes the cathode). Prior art fluorine cells may typically have upto 36 separate anodes. Thus, in the present invention, each fluorinecell or cassette may have, for example, 6, 12 or 24 anodes dependingupon the customer's fluorine requirements.

It is also further intended that the removed fluorine cassette bereplaced with another substantially identical fluorine cassette suchthat the fluorine generating capacity of the apparatus is notsignificantly impaired.

The apparatus according to the present invention provides a selfcontained fluorine generation system which has sufficient fluorinegenerating capacity for the plant, for example as a cleaning gas for CVDchambers or tools, to which it is connected such that the total fluorinedemand may be met by less than the total number of individual fluorinecassettes in the apparatus. Thus, if one fluorine cassette, for example,requires repair or maintenance or servicing then the apparatus maycontinue to generate the total fluorine requirement without shut-down ofthe fluorine generating apparatus according to the present invention orother interruption. As noted above, the removed fluorine cassette may beimmediately replaced with a substantially identical (in importantmaterial and dimensional respects) fluorine cassette so that totalpotential fluorine generating capacity is not impaired at all. Forexample, for apparatus having three fluorine generating cassettes, thenormal average output of the three cassettes will be less than 66% ofpeak output of each cassette at peak demand. Therefore, if one cassetteneeds to be removed for any reason, the two remaining cassettes will beable to meet the total peak fluorine demand of the application which theplant is supplying.

Thus, a single point failure of a fluorine cassette of the apparatus ofthe present invention does not result in shut-down of the apparatus as awhole nor a reduction in the ability to supply the peak demand offluorine.

As noted above, fluorine generating cells have traditionally beenmaintained, serviced or repaired in-situ which has necessitated closingdown the cell and dismantling it in-situ which, because of the extremelyhazardous nature of fluorine gas and the electrolytes employed, hasmeant that all but essential personnel, suitably attired, must beevacuated from the area whilst the work is undertaken. Usually thedown-time has been several days to complete the work. This has meantdisruption and lost production time in the plant which is fed by thefluorine apparatus.

With the apparatus of the present invention, it is intended that thefluorine cassette requiring work is isolated electrically and by valvemeans insofar as the fluorine gas aspect is concerned, removed from theapparatus and transported, by truck for example, to a remote site wherethe required work is undertaken. However, a replacement fluorinecassette, held as a spare in store on-site, may be immediately installedin the apparatus. Thus, there are no time constraints on the timerequired for the removed cell to be brought operational again; the cellmay be worked on at a site which is properly equipped for such work; nohazard is produced in respect of those people working at the plant fromwhere the fluorine cassette was removed; and, there is no lostproduction time at the plant.

The apparatus according to the present invention may be relatively smallin terms of overall size. It may, for example, as noted hereinabove,approximate the size of a standard ISO container which are usedinternationally for shipping and transportation of many types of goods.Such containers have an overall size of about 2.44 m wide×2.44 mhigh×6.5 m long (or about 8′ wide×8′ high×20′ long in Imperialmeasurements). Thus, the fluorine generating apparatus of the presentinvention may have a small footprint and be easily situated in aconvenient location within a customer's production plant.

The apparatus of the present invention comprises an integrated,self-contained fluorine generating plant which may be shipped as a unitby land or sea. The apparatus according to the present invention whenbased on an ISO type container may comprise up to three fluorinecassettes, at least one fluorine purification cassette and at least onefluorine compression and storage/buffer cassette.

One embodiment of the present invention comprises a fluorine generatingapparatus having three fluorine cassettes, two fluorine purificationcassettes, a fluorine compression and storage cassette and otherassociated equipment all enclosed within the outer dimensions of astandard ISO container. However, there is another aspect of shipping andthat is the shipping of spare or replacement fluorine cassettes; astandard ISO container may accommodate up to eight fluorine cassettesbased on a width of approximately 0.74 m, however, this is given as anexample only as the individual fluorine cassettes may be constructed ina range of individual sizes. The cassettes of the apparatus of thepresent invention are self-contained shipping packages by virtue oftheir outer enclosure which is completely panelled making the cassette asealed unit and needing no further packaging or protection for shipping.

Although the fluorine cassette for a particular apparatus according tothe present invention may be formed to a particular set of outerdimensions and have services connections and fluorine outlets andhydrogen outlets and the like in predetermined positions so as to ensureinterchangability, the actual fluorine cell within the fluorine cassettemay be varied to suit the fluorine demand of a customer. For example,the fluorine cell may initially have a fluorine generating capacity of 0to 385 g/hr and, as demand increases, may be changed for a fluorine cellhaving an output capability of 0 to 700 or 0 to 1400 g/hr, for example.Thus, the apparatus according to the present invention may be upgradedas fluorine need increases.

The fluorine cassettes may be installed within an overall apparatusenclosure which may house common services for all the fluorine cassetteswithin that apparatus enclosure. Such common services may include fluidpiping, electrical cable trunking and electrical/instrument wiringharnesses.

Hazardous fluid services to the fluorine cassettes are provided withisolation valves. Preferably, the hazardous fluid services to thecassettes are provided by double isolation valves which have a vacuumconnection in between. Thus, the two isolation valves may be closed andvacuum applied to remove hazardous contents before breaking theconnection between the fluorine cassette and the associated piping priorto removal of the fluorine cassette from the apparatus. The vacuumextraction system for removing hazardous materials is preferablyconnected to a scrubbing system for removing and neutralising harmfulmaterials.

Non-hazardous fluid services may be provided by means of quick-connectcouplings, for example.

Once the fluorine cassette is disconnected from the apparatus it iscompletely sealed from the ambient environment and constitutes no dangerto personnel. Similarly, a new replacement fluorine cassette is alsoharmless until it is connected into the apparatus and the isolationvalves opened to permit flow of hazardous fluids.

Spare fluorine cassettes may be stored on-site to enable quickreplacement. Similarly, removed cassettes may be safely stored on siteuntil they can be removed to a remote repair or servicing facilityoff-site, for example.

The fluorine cassettes may be provided with means to facilitate movingthe cassette into and out of the apparatus enclosure. Suitable means maycomprise wheels, for example.

In a preferred embodiment of the present invention the fluorine cassetteis itself be provided with a self-contained enclosure around thefluorine generating cell or cells such that any leakage of fluorine gasfor example is contained within the enclosure. More preferably, thecassette enclosure may be connected to a vacuum extraction system whichis provided with scrubbing means to remove harmful chemicals.

In the apparatus of the present invention each cassette has its ownenclosure provided by a strong metal frame and panelling. When installedin the apparatus the enclosure is connected to extraction and scrubbingmeans to deal with any potential leaks, and therefore, provides a sealedprotective enclosure. When the cassette is removed from the apparatus,the enclosure provides the dual advantage of constituting protection forthe cassette during shipping without further packaging and protectionfor people handling the cassette.

In a preferred embodiment of the apparatus according to the presentinvention there is also provided at least one, preferably two fluorinepurification cassettes through which the output of the fluorinecassettes is passed so as to remove unwanted particulate material orundesirable gaseous contaminants therefrom before it reaches any processequipment for which it is destined. The purpose of having two suchcassettes is to allow for one cassette needing repair or replacementduring fluorine production. Such unwanted material may include hydrogenfluoride, for example, carried over from the electrolyte in the fluorinestream and which may be passed through a sodium fluoride trap, forexample. Carbon tetrafluoride formed by reaction of fluorine with thecarbon anodes may be removed by an appropriate known adsorption system.

The fluorine purification cassette may also be isolatable and easilyremoved from the apparatus for repair and servicing in a similar mannerto the fluorine cassettes. Thus, such isolation may be accomplishedpreferably by double isolation valves with an intervening vacuumextraction facility as with the fluorine cassettes.

In a further preferred feature of the apparatus according to the presentinvention, there may also be provided a fluorine buffer cassetteconnected in the fluorine line downstream of the fluorine purificationcassette. In effect, the buffer cassette collects purified fluorinebeing generated and holds it in tanks so as to provide a fluorinereservoir to smooth out fluctuations in supply and provide fluorine at aconstant pressure.

The apparatus according to the present invention may all be housedwithin a main enclosure framework which is provided with suitablepanelling so as to effectively render the enclosure sealed to theoutside ambient atmosphere. Further preferably, the main enclosure isprovided with evacuation means so that any leakage is removed and doesnot contaminate the surrounding area.

The evacuation system may be connected to suitable scrubbing means forthe removal and safe disposal of any harmful substances.

The main enclosure may also preferably be provided with all of thenecessary electrical power supply and electrical control systems inknown manner so as to effect electrolysis of the hydrogen fluorideelectrolyte to generate fluorine.

According to a preferred embodiment of the apparatus according to thepresent invention the framework of the fluorine cassette may be utilisedas the cathode connection of the fluorine cell or cells within thecassette thus, the mere installation of the cassette within the mainenclosure effects the necessary cathode connection to the electrolyticcell or cells within the cassette.

Preferably, the apparatus according to the present invention is furtherprovided with purging means to remove potentially reactive fluids suchas moisture, for example, from piping before fluorine is introduced.Such purging means may comprise valve means connected to apparatuspiping for the introduction of nitrogen, for example, into the piping soas to purge oxygen, for example, from the piping.

In an alternative embodiment of the apparatus according to the presentinvention, each individual fluorine cassette may be provided with thenecessary facilities so as to enable the apparatus to continue tofunction even if some “centralised” apparatus services should fail. Inthis alternative embodiment, the self-contained cassette enclosure mayalso be provided with a D.C. power supply unit for electrolysis,fluorine purification and compressor means and a fluorine storagetank/buffer facility. The cassettes are of relatively large volume andthe fluorine generating unit is housed in the lower portion of thecassette enclosure leaving sufficient room to accommodate the additionalfacilities; Connections to the storage tank/buffer facility may also beby double isolation valves. In this alternative embodiment,disconnection of the cassette from the apparatus enclosure would be fromthe output of the fluorine tank/buffer unit as the fluorine cell per se,purification and compression facilities are upstream therefrom.

According to a second aspect of the present invention, there is provideda method for the operation and maintenance of apparatus for producingfluorine by the electrolysis of hydrogen fluoride, the method comprisingthe steps of: providing a plurality of fluorine generating unitsoperably connected to a fluorine gas distribution system for the remoteuse and consumption of said fluorine; providing means for isolating anindividual fluorine generating unit from said fluorine gas distributionsystem and from each other; and providing means for the disconnectionand removal of said isolated fluorine generating unit from saidapparatus without interruption of supply of fluorine from remainingfluorine generating units.

The same definition relating to fluorine generating units is utilised inthe method according to the present invention as in the apparatusaccording to the first aspect of the invention.

In order that the present invention may be more fully understood,examples will now be described by way of illustration only withreference to the accompanying drawings, of which:

FIG. 1 shows a schematic cross sectional representation of a prior artfluorine producing cell;

FIG. 2A shows a front elevation (without panelling) of a firstembodiment of apparatus according to the present invention;

FIG. 2B shows an end elevation of the apparatus of FIG. 2A;

FIG. 3 shows a side elevation of a fluorine cassette of FIG. 2 withoutenclosure panelling;

FIG. 4 shows a front elevation of the fluorine cassette of FIG. 3without enclosure panelling;

FIG. 5 shows a side elevation of a fluorine purification unit withoutpanelling of the apparatus of FIG. 2;

FIG. 6 shows a front elevation of the purification unit of FIG. 5;

FIG. 7 shows a side elevation of a buffer unit without enclosurepanelling of the apparatus of FIG. 2;

FIG. 8 shows a front elevation of the buffer unit of FIG. 7; and

FIG. 9 which shows a perspective view of a second embodiment ofapparatus according to the present invention but with main and otherenclosure panelling removed in the interests of clarity.

Referring now to FIG. 1 which is intended only to explain the basicprinciples of the production of fluorine by electrolysis. A fluorinegenerating cell is shown schematically in section at 10. The cellcomprises a containment vessel 12 which may or may not also constitutethe cathode of the cell; in this case a separate cathode is shown at 14.The top 16 of the vessel 12 is closed save for outlets 18, 20 havingvalve means for fluorine and hydrogen, respectively. The vessel 12contains an electrolyte 22 of hydrogen fluoride in a molten potassiumfluoride salt. A separating skirt 24 depends from the vessel top wall 16and its lower end 30 extends below the surface 32 of the electrolyteeffectively dividing the volume above the electrolyte surface into twoseparate chambers 34, 36 for hydrogen and fluorine, respectively. Ananode 38 generally of high-density isotropic carbon extends into theelectrolyte 22 and generally extends below the lowermost extent of theskirt 24 although this may not always be the case. The vessel 12 isgenerally provided with means (not shown) to heat and melt theelectrolyte as it is solid at room temperature. Generally, theelectrolyte is maintained in the range from 80 to 100° C. by the heatingmeans when the cell is quiescent. During electrolysis heat is generatedand is generally necessary to cool the electrolyte by suitable coolingmeans. Any suitable heating means may be used and, for example, maycomprise tube heaters extending into the vessel and passing through theelectrolyte, an electrically heated blanket around the vessel or a steamjacket around the vessel. A suitable power supply 40 is provided toeffect electrolysis of the electrolyte. Generally, the voltage isrelatively low at about 6 to 9 volts but the current is high at about500 to 2400 amps depending upon the number of anodes in the cassette.

The electrolysis reaction is:2HF→F₂+H₂

The amount of fluorine generated is in direct proportion to the appliedcurrent. The gases, fluorine and hydrogen, rise substantially verticallyfrom the anode and cathode surfaces into their respective compartmentsabove the electrolyte surface 32. The electrolyte temperature isregulated as noted above and the composition and level is controlled bythe addition of anhydrous hydrogen fluoride.

Turning now to FIGS. 2 to 8 and where the same features are denoted bycommon reference numerals. Apparatus according to a first embodiment ofthe present invention is shown at 100; the apparatus comprises a mainenclosure framework 102 which has removable panelling (not shown) toform a main sealed enclosure 104 in use and which sealed enclosure isconnected to a vacuum extraction system (not shown) via a manifold 124which in turn is connected to a scrubbing system (not shown) toneutralise harmful chemicals. Within the main enclosure 102, 104 arehoused three fluorine cassettes 106, 108, 110 which are allsubstantially identical in the sense that each may be substituted foranother and have the same positioning of connection fittings such asisolation valves, pipes, pipe fittings, electrical services and thelike, for example. The fluorine cassettes are connected to a fluorinegas manifold 114 for off-take of fluorine process gas as it is producedby the fluorine cassettes via a fluorine gas standpipe 116 connected tothe fluorine compartments of the electrolytic cells in the cassette (seeFIG. 1 above and also below for more detailed description of thefluorine cassette). The standpipe 116 is connected to the manifold 114via double isolation valves 118, 120, the intervening space betweenwhich is connected to the vacuum extraction fluorine manifold 124 whichin turn is connected to the scrubbing system (not shown) forneutralising any harmful gases. The hydrogen produced duringelectrolysis is piped away via a standpipe 130 on each cassette, thestandpipes being connected via flanged joints 132 to a hydrogen gasmanifold 134 which conducts the hydrogen away for either treatment orburning-off as appropriate. All of the pipe work through which fluorineflows is connected via suitable valve means (not shown) to a source of apurging gas (not shown) such as nitrogen, for example, to permit purgingof oxygen and/or moisture from the pipe work prior to the introductionof fluorine.

Each individual fluorine cassette 106, 108, 110 comprises a cassetteenclosure frame 140 which is able to be split horizontally into twoparts: a lower part 142 which houses the fluorine generating cells 144;and, an upper part 146 which houses power supplies for electrolysis andthe like. Splitting of the cassette enclosure permits easy access to thefluorine generating cells after removal of the cassette from the mainenclosure 102, 104. To aid mobility, the fluorine cassette is providedwith wheels 148 to facilitate removal from the main enclosure 102, 104.The cassette shown in FIGS. 3 and 4 has one cell producing fluorine but,however, the cell may contain 6, 12 or 24 anodes depending upon requiredfluorine generating capacity as explained hereinabove. The total outputof fluorine in each cassette is conducted internally to the singlefluorine off-take pipe 116 having the double isolation valves 118, 120.Similarly, all hydrogen generated during electrolysis is conducted tothe single off-take standpipe 130. The fluorine generating cells 144have a common containment vessel 150 fabricated from steel and forms thecell cathode and which is welded to the lower part 142 of the enclosureframe 140. Thus, the enclosure frame forms the cathode connection forthe whole cassette. Each cassette may have its own D.C. power supply 152and control system 154, however, the power supply and control system forall cassettes may be centralised in the main enclosure 104. The upperpart 160 of the main enclosure 102, 104 houses busbars and main powersupplies (not shown) and the like to which each of the fluorinecassettes are connected on being installed into the main enclosure bymeans of plug-in electrical connectors (not shown) to a junction box158.

As noted above the enclosure frame may form the cathode connection ofthe apparatus of the present invention. Since the frame is the cathodeit also carries the current which may be up to about 2400 amps with a 24anode cassette. Thus, the frame is made of substantial section materialin order to prevent undesirably high temperatures being reached due toresistance heating. The cathode connection is made at 0 volts relativeto earth whilst the anode connection is at 6 to 9 volts. Use of theenclosure frame as the cathode connection and current carrier enablesthe apparatus to be more economically made with a strong frame due tothicker section materials and, without unnecessary extra copper cablingto make the cathode conductors. Since the frame is a 0 volts relative toearth, the apparatus is electrically very safe.

The total fluorine output of the fluorine cassettes 106, 108, 110 isconnected to a fluorine purification cassette 170 through which thefluorine is passed to remove particulate material such as hydrogenfluoride or other electrolyte constituents which have been carried overby the fluorine stream and contaminants formed during electrolysis. Thepurification cassette is shown in more detail in FIGS. 5 and 6. Thepurification cassette comprises a container 172 housing chemical trapsand filters (not shown) for removing unwanted material from the fluorinestream in known manner. The purification cassette 170 has an enclosureframework 174 enclosing the container 172 and, in similar manner to thefluorine cassettes has a double isolation valve 178, 180 to permitinstallation in and removal of the purification cassette from theapparatus when required. The unit is provided with wheels 180 to aidmoving.

The purified fluorine gas is passed from the cassette 170 to a fluorinecompression cassette 190 shown in FIGS. 7 and 8. The compressioncassette comprises, in this example, three holding tanks 192 having atotal capacity of 650 litres and able to safely withstand 5 Bar pressureof fluorine although such pressures are not generally employed withfluorine in the interests of safety. The output of purified fluorinefrom the purification cassette 170 is fed to the compression cassettepump 194 and via a pressure controller 196 to the holding tanks 192. Thecompression cassette 190 holds a reserve of fluorine such that if forany reason the apparatus had to be closed down in respect of fluorineproduction for a period of time, for example, to change the purificationcassette 170, then there would be a reserve of fluorine to continue withthe process requirements until fluorine production can be resumed. Thecompression cassette also smoothes out fluctuations in fluorineproduction so that fluorine may be supplied to the process plant at aconstant pressure, for example. In similar manner to the fluorinecassette and the purification cassette, the compression cassette has anenclosure frame 200 and wheels 202. The compression cassette isconnected to the fluorine manifold 114 again by double isolation valves(not shown) as with the fluorine cassette and the purification cassette.The fluorine output is via a second pressure controller 198 to thefluorine manifold 114 and then to the process plant where the fluorineis to be used.

As may be seen from FIGS. 2A, 2B, 3 and 4 the fluorine cassette 106, forexample, may be installed in and removed from the main enclosure 102,104 without need to interfere with the two other cassettes 108, 110which may continue to provide fluorine for the required processesoperated externally to the apparatus. The fluorine generating capacityof the apparatus 100 is calculated such that the total processrequirements of the plant being served may be met, for example, by anytwo of the three cassettes of the apparatus described thus allowing onecassette to be redundant or removed and replaced as required.

In the example described above the apparatus 100 approximates less thanthe size of an ISO container in terms of length, and therefore, may beeasily transported by land or sea. It is further possible to provide theapparatus as described above but somewhat larger but still within thefootprint of a standard ISO container and having empty spaces therein soas to accommodate additional fluorine cassettes for example to provideexpansion of fluorine generating capacity as need arises. The emptyspaces may be provided with the necessary valves and pipe-connection tothe manifolds etc so that an additional fluorine cassette may merely beconnected into the system as with the existing cassettes.

The main enclosure frame. 102 is provided with removable panelling so asto effect a substantially sealed enclosure, in use, to the egress offluorine, for example. The enclosure is connected to the site extractionand scrubbing system to neutralise harmful chemicals. Furthermore, eachfluorine cassette, purification cassette and compression cassette issimilarly equipped, in use, with panelling on the frames 140, 174 and200 so as to form substantially sealed sub-enclosures within the mainenclosure 102, 104, the sub-enclosures also being connected to the siteextraction and scrubbing system.

FIG. 9 shows a single perspective view of a fluorine generating plant300 according to a second embodiment of the present invention. In termsof the capacity and capability to produce, process, control and storefluorine the apparatus of FIG. 9 is similar to that described withreference to FIGS. 2 to 8 of the first embodiment. The apparatus 300again has a main enclosure framework 302 which is provided withpanelling (not shown) to form a substantially sealed enclosure. Threefluorine cassettes 304, 306, 308 are provided, each having their ownenclosure frameworks 310, 312, 314 with panelling (not shown) and eachbeing individually isolatable and removable by means of valves (notshown), as with the first embodiment. Fluorine is passed through apurification cassette comprising a duty purifier 320 and a standbypurifier cassette 324 and then to a compression cassette comprising aplurality of storage tanks 326 by means of duty and standby compressors328, 330. The fluorine is then piped to CVD tools, for example, for use.A supply of liquid hydrogen fluoride is held in a tank 332. A hydrogenfluoride vaporiser 334 vaporises liquid hydrogen fluoride from the tank332 and supplies it to the cassettes 304, 306, 308 to maintain aconstant concentration of electrolyte. A fluorine abatement cassette 340is provided to remove solids from the fluorine supply, to removefluorine from pipework when a cassette is being changed for servicing orrepair, for example, and extraction of fluorine not destined forcustomer process use. The apparatus of FIG. 9 has all of the pipeworkpurging systems, safety extraction and scrubbing systems of the firstembodiment.

1. Apparatus for the generation of fluorine by the electrolysis ofhydrogen fluoride, the apparatus comprising: a plurality of individualfluorine generating cassettes; said individual fluorine generatingcassettes being operably connected to a fluorine gas distribution systemfor the remote use and consumption of said fluorine gas; said fluorinegenerating cassettes being individually isolatable from said gasdistribution system and removable from the apparatus for remotemaintenance, as hereinbefore defined. 2-23. (canceled)
 24. The apparatusaccording to claim 1 wherein said fluorine generating cassettes areconnectable to the apparatus by a valve mechanism for the isolation anddisconnection of said fluorine generating cassettes from the apparatus.25. The apparatus according to claim 24 wherein said valve mechanismincludes a double isolation valve having a space therebetween, saidspace connectable to an extraction and scrubbing system.
 26. Theapparatus according to claim 1 wherein the fluorine generating cassettesare installable within a common apparatus main enclosure.
 27. Theapparatus according to claim 1 wherein all fluorine generating cassettesare substantially identical to each other.
 28. The apparatus accordingto claim 1 wherein said fluorine generating cassettes are provided withwheels.
 29. The apparatus according to claim 1 wherein each fluorinegenerating cassette is provided with an enclosure.
 30. The apparatusaccording to claim 26 wherein said main enclosure is connectable toextraction equipment and to a scrubbing system.
 31. The apparatusaccording to claim 29 wherein each fluorine generating cassetteenclosure is connectable to extraction equipment and to a scrubbingsystem.
 32. The apparatus according to claim 29 wherein a fluorinegenerating cell within said fluorine generating cassette is fixed tosaid enclosure such that said enclosure provides a cathode connection tosaid cell.
 33. The apparatus according to claim 32 wherein saidenclosure includes a framework having panelling.
 34. The apparatusaccording to claim 32 wherein said cathode connection is at 0 voltsrelative to earth.
 35. The apparatus according to claim 1 furthercomprising at least one fluorine purification cassette through which thefluorine output of said fluorine generating cassettes is passed.
 36. Theapparatus according to claim 35 further comprising at least one fluorinebuffer cassette connected in a fluorine line downstream of said at leastone fluorine purification cassette.
 37. The apparatus according to claim36 wherein said buffer cassette holds compressed fluorine.
 38. Theapparatus according to claim 1 further including purging means to removepotentially reactive fluids from piping before fluorine is introducedthereinto.
 39. The apparatus according to claim 1 wherein the apparatusis transportable as a unit by land or sea
 40. The apparatus according toclaim 39 wherein the overall size of the apparatus is at most that of astandard ISO container.
 41. The apparatus according to claim 1 whereineach of said individual fluorine generating cassettes are furtherprovided with a power supply unit at least for electrolysis, fluorinepurification, fluorine compression and a fluorine storage tank/buffer.42. A method for the operation and maintenance of an apparatus forproducing fluorine by the electrolysis of hydrogen fluoride, the methodcomprising the steps of: providing a plurality of fluorine generatingcassettes operably connected to a fluorine gas distribution system forthe remote use and consumption of the fluorine; isolating any individualfluorine generating cassettes from the fluorine gas distribution systemand from each other; and disconnecting and removing the isolatedfluorine generating cassette from the apparatus without interruption ofsupply of fluorine from remaining fluorine generating cassettes.
 43. Themethod according to claim 42 further comprises the step of providing thefluorine generating cassettes with sufficient fluorine generatingcapacity such that a total demand for fluorine may be met by less thanthe total number of fluorine generating cassettes within said apparatus.44. The method according to either claim 42 further comprises the stepof removing an individual fluorine generating cassette from theapparatus and taking said cassette to a remote site for maintenancewhile still maintaining fluorine output to meet demand.
 45. The methodaccording to claim 42 further comprising the step of providing eachindividual fluorine generating cassette with a power supply at least forelectrolysis, fluorine purification, fluorine compression and a fluorinestorage tank/buffer.